1. Field of the Invention
This invention relates to a device for developing an electrostatic image by the use of a one-component magnetic developer.
2. Description of the Prior Art
The cascade developing method and the magnet brush developing method have heretofore been widely used to develop electrostatic images formed by various known electrophotographic methods or electrostatic recording methods. These two developing methods have a common feature in that they use a two-component developer consisting of a suitable ratio of a mixture of minute coloring particles called toner and relatively rough particles called carrier and that the toner frictionally charged by the frictional contact thereof with the carrier is selectively deposited on the electrostatic image. However, these two developing methods have serious problems coming from the above-noted common feature.
A first problem is one regarding the supply of the developer. In the above-mentioned two-component developer, it is necessary in order to prevent fog during development and to provide a proper image density that the ratio of mixture of the toner and the carrier is always at a certain constant value. On the other hand, it is the toner only that is consumed by being deposited on an electrostatic image carrier during the development. Therefore, a complicated toner supply device is necessary to maintain the constant ratio of mixture at all times, but no fully satisfactory toner supply device has been put into practice.
A second problem is concerned with the deterioration of the developer. If the developer is used for a long time, the coating of the resin component of the toner is formed on the surface of the carrier particles and the frictional charging characteristic between the toner and the carrier becomes deteriorated. Therefore, the developer itself must cumbersomely be replaced by fresh one.
The above-noted problems may be avoided by using a one-component developer which does not contain the above-mentioned carrier by which the toner is frictionally charged.
Various devices for developing electrostatic images by the use of a one-component developer are also known and it is usual with the devices of this type to use a magnetic developer as the one-component developer, which mangetic developer comprises coloring particles (toner particles) containing fine particles of a magnetic material such as magnetite or the like. To take out such one-component magnetic developer from its supply station and convey it through a developing station, use has often been made of a mechanism having a non-magnetic cylindrical member opposed to the electrostatic image carrier in the developing station with a minute clearance interposed therebetween, and a multi-pole magnet member disposed within the hollow of the cylindrical member. The non-magnetic cylindrical member takes the supply of the one-component magnetic developer on the peripheral surface thereof and carries the developer layer on the peripheral surface; to convey the magnetic developer layer carried on the peripheral surface of the non-magnetic cylindrical member from its supply position through the developing station to its containing position, the conventional devices have adopted one of various methods, namely, a method whereby the multi-pole magnet member is unrotatably fixed while the non-magnetic cylindrical member is rotatively driven in the direction of conveyance of the developer, or a method whereby the non-magnetic cylindrical member is unrotatably fixed while the multi-pole magnet member is rotatively driven in the direction opposite to the direction of conveyance of the developer, or a method whereby the non-magnetic cylindrical member is rotatively driven in the direction of conveyance of the developer while the multi-pole magnet member is rotatively driven in the direction opposite to the direction of conveyance of the developer.
On the other hand, in the developing devices using the one-component magnetic developer, the clearance between the cylindrical member and the electrostatic image carrier in the developing station is usually narrow. However, in the conventional developer conveyance method, the conveyed developer is liable to be pressed in the narrow interval between the cylindrical member and the electrostatic image carrier. When such pressing phenomenon occurs, the developer is liable to adhere to the peripheral surfaces of the cylinder and the electrostatic image carrier to damage the electrostatic image carrier and this not only results in degraded quality of the developed image and hampered reproducibility of the image but also results in reduced or no disturbance movement of toner particles in the developer pressing portion even if no adherence of the developer takes place, and this in turn results in an inconvenience that the electrostatic image is not developed at all or details thereof are not developed to reduce the development density of the wider area image portion. Such inconvenience also occurs to a device in which the electrostatic image carrier is moved in the direction opposite to the direction of conveyance of the developer in the developing station, and is particularly pronounced for the devices in which the electrostatic image carrier is moved in the same direction as the direction of conveyance of the developer, or for weakly magnetic developer, or under high-temperature and high-humidity conditions.
Further, in order to obtain a developed image having a higher density and having more faithfully reproduced image details, it is desirable to lengthen the developing time by forming a pool of developer (an accumulation of developer forming a contact region upstream by an amount corresponding to the length of a downstream contact region, in the region in which the developer contacts the electrostatic image carrier upstream of the minimum clearance portion in the developing station between the electrostatic image carrier and the developer carrier means such as non-magnetic cylinder with respect to the path of conveyance of the developer) on the inlet side (with respect to the path of conveyance of the developer) of the interval portion between the non-magnetic cylinder and the electrostatic image carrier, namely, the developing station and widening the developing width (the length of contact of the developer with the electrostatic image carrier with respect to the direction of movement of the electrostatic image carrier). However, where such a pool of developer is formed, the above-mentioned pressing phenomenon is more liable to occur in the conventional developer conveying method and if not so, the degree of disturbance movement of the toner particles in such pool portion becomes low and the widened developing width does not result in a correspondingly increased developing effect.